Method of operating photo-electric cells having auxiliary anodes



Jan. 8, 1935. EN 1,986,806

' METHOD OF OPERATING PHOTd-ELECTRIC CELLS HAVING AUXILIARY ANODES FiledMay 15. 1950 I 5: 1 I EH2.

SENSITIVITY SENSITIVITY SENSITIVITY M Patented Jan. 8, 1935 UNITEDSTATES METHOD OF OPERATING PHGTO-ELECTRIC CELLS HAVING AUXILIARY ANODESHeinrich H. Geffcken, Leipzig, Germany, assignor to Radio PatentsCorporation, New York, N. Y., a corporation of New York Application May13, 1930, Serial No. 452,05

In Germany May 21,

7 Claims.

This application relates to photoelectric cells, and more particularlyto gas filled photoelectric cells.

It has already been suggested to provide a plurality of anode electrodesin a photoelectric cell for reducing the reaction on the operatingcharacteristic of the cell due to terminal voltage variations in theoutput circuit. These conditions are analogous to the conditionsprevailing in the electron tube, where it is possible to minimize theeffect of the anode voltage variations by means of screening orsuppressor electrodes. However. it was discovered that theseconsiderations cannot be applied offhand from the electron tube to thegas filled photoelectric cell. If the photoelectric current of suchcells is plotted against the anode voltage with the ,illumination beingkept constant, it will be observed that the auxiliary electrode,arranged between the working anode and the light sensitive electrode,has practically no influence on the operating characteristic of thecell. Under circumstances, even a decrease in sensitivity is observed,and one might be inclined to completely reject the use of an auxiliaryanode.

The object of the present invention is, therefore, to provide means foreffectively utilizing an auxiliary electrode in a photoelectric cell.

A more specific object of the invention is to provide an auxiliaryelectrode in a photoelectric cell and means for applying proper biasingpotential thereto, to maintain substantially uniform sensitivity of thecell within a predetermined operating range. Another object of theinvention is to provide an auxiliary anode for a photoelectric cell withproper biasing potential applied thereto, to produce a substantiallydrooping shape of the sensitivity characteristic of the cell.

These and still further objects and aspects of the invention will becomemore apparent as the detailed description proceeds, taken in referenceto the accompanying drawing, of which Fig.1 shows a simple circuitdiagram for car.- rying the invention into effect.

Figs. 2-4 illustrate theoretical curves-explaining the operation inaccordance with the invention.

The invention is based on the recognition that it is of primaryimportance for the effectiveness of the auxiliary anode in which part ofthe space, enclosed by the cell, ionization takes place under a givencondition. According to the invention,

the biasing potential of the auxiliary anode is chosen of such highvalue that the characteristic curve for the main anode (anode currentplotted against anode potential with illumination being kept constant)exhibits a decrease of its steepness in the neighborhood of the biasingpotential value for the auxiliary anode. It has been observed that thispeculiar phenomenon may be easily reprcduced by choosing acorrespondingly high potential of the auxiliary anode. It is evenpossible, by choosing very high biasing potentials for the auxiliaryanode, to effect a decrease of the steepness of the characteristic curveto such an extent that the result is a negative or droopingcharacteristic, i. e. that the anode current decreases with increasingpotential within a certain voltage range. As is obvious, theseconditions are basically different from the principle 01 the screengrid, as used in ordinary electron tubes. The invention also differsfrom the socalled dynatron principle, as there is no secondary electronemission from the main anode electrodc.

Referring more particularly to Fig. 1 of the drawing, the photoelectrictube shown includes a light sensitive layer 1 of substantially flatshape, which may be applied directly to the glass wall of thephotoelectric tube, as shown, an auxiliary netlike anode 2 disposedopposite the light sensitive layer at a distance of about 3-6 mm., and anetlike main anode 3 also disposed at about 3-6 mm. distance from theauxiliary anode. The potential for both anodes may be adjusted by meansof a potentiometer 4, provided with slidable tap connections andconnected to a potential supply source as indicated by the plus andminus signs in the drawing. The circuit for the auxiliary anodefurthermore includes a safety or ballastresistance 5, for instance ofabout 50,000 ohms, and the circuit for the main anode 3 includes theworking resistance 6 of, for instance, 2 meg ohms. With varyingillumination of the cell a varying potential drop occurs at the workingresistance 6, causing a shift of the anode terminal voltage in the caseof ordinary gas filled photoelectric cells, thus resulting in anundesired decrease of the sensitivity of the cell with varyingillumination.

Curve '7 according to Fig. 2 illustrates the sensitivity characteristicof an ordinary gas filled photoelectric cell, including a single anode,the sensitivity E (anode current plotted against anode voltage withconstant illumination) being plotted against the anode potential Va.This illustrates the well known curve, assuming almost infinitesteepness at the ignition point of the cell whereby an independent glowdischarge unaffected by the illumination of the tube is produced. It isobvious that the sensitivity of such a cell is highly dependent upon theoperating potential. Curve 8 according to Fig. 2 in turn illustrates thecharacteristic of a photoelectric cell including two anodes andconnected in a circuit as shown by Fig. 1. The sensitivity E (main anodecurrent per unit of illumination) is plotted against the anode potentialVa. As is seen, this curve practically does not difier from a normalphotoelectriccell with a single anode.

However, if, in accordance with the invention,

Gil

the potential of the auxiliary anode is gradually increased, sensitivitycurves are obtained as shown by Figs. 3 and 4. In the case of curve 9,according to Fig. 3, the auxiliary anode carries a biasing potential ofabout volts, and in the case of curve 10, according to Fig. 4, thebiasing potential for the auxiliary anode is about volts. It is readilyseen that with the proper choice of the biasing potential of theauxiliary anode, in accordance with the invention, within a certainrange suddenly a strong decrease of the steepness of the anode currentor the sensitivity, respectively, sets in. In this manner, an operatingrange for the photoelectric cell is secured wherein comparatively smallvariations of the sensitivity take place, due to the fact that theauxiliary anode reacts on the output current in the desired manner tomaintain uniform operating conditions.

The explanation for this astonishing phenomenon may be given in thatwith suificiently high biasing potential of the auxiliary anodeionization at first takes place in the space between cathode 1 andauxiliary anode 2. If the potential of the main anode 3 is increased andbecomes equal to the potential of the auxiliary anode 2, the region ofionization is gradually shifted into the space between the auxiliaryanode 2 and main anode 3. The electrons arriving from the cathode 1 areshooting from space 1, 2 through the meshes of the auxiliary anode 2into the space 2, 3, where they will effect their ionization action. Dueto this displacement of the ionizing region from space 1, 2 into space2, 3, a large part of the gaseous ions is brought into a. field of verylow potential drop or correspondingly increased probability ofrecombination. Besides many of the ions will be neutralized by theauxiliary anode. Thus a decreased amount of ions will reach the cathode,whose electronic emission is no longer increased to the same extent, byionic bombardment, as previously. The above explanation is confirmed byexperiments during which a transition of the auxiliary anode currentfrom a positive to a negative sense was observed within the abovevoltage range. Regarded diagrammatically, this phenomenon appears to bedue to a transitionof the sensitivity characteristic from one position(corresponding to the auxiliary anode) into another position(corresponding to the main anode), indicated by the dotted linesaccording toFig. 4.

It has been found advisable to use an operating potential for the mainanode, which is higher or equal to the potential'of the auxiliary anode.In accordance with the invention the anode potential, dependent on theaverage value of the illumination at which the cell operates, isadjusted to such a value with the potential variations during operationremaining within the region as specified according to the invention. Inthis manner, distortions are prevented to a large extent, and thesensitivity of the cell maintained at a substantially uniform value.

In some cases it is preferable to utilize the negative characteristicwhich may be obtained by providing sufficiently high biasing potentialfor the auxiliary anode, such as for directly producing a trigger actionas is well known in connection with gaseous relays. In this manner, theinvention makes it possible to obtain sudden and abrupt changes of theanode current, whenever the illumination of the cell increases ordecreases by a predetermined amount, for producing a release such as ofa mechanical relay or the like in various manners.

It is obvious that the invention as described, is subject to manyvariations and modifications, coming within its broader scope, ascomprised by the appended claims.

What I claim is:

1. In combination, a photoelectric tube comprising, a vessel filled witha gaseous atmosphere, a light sensitive cathode and an anode therein, anauxiliary electrode, means for applying operating potential, furthermeans for applying a biasing potential to said tube insuflicient toproduce an independent glow discharge therethrough, to said auxiliaryelectrode, of a critical value to secure a drooping shape of thesensitivity characteristic of the tube for a critical operating range.

2. An electrical system as claimed in claim 1,

in which a common potentiometer is provided for securing operating andbiasing potential for said tube, and a ballast resistance in theconnection from said potentiometer to said auxiliary electrode.

3. In combination with an electrical system, a

photoelectric tube comprising, a vessel filled with a gaseousatmosphere, light sensitive cathode and anode electrodes within saidvessel, an auxiliary anode, a working impedance in series with saidanode electrode, means for applying operating potential to said tubeinsufficient to produce'an independent glow discharge therethrough andfurther means for applying'a biasing potential to said auxiliaryelectrode, of a critical value to se cure uniform sensitivity of saidtube over a critical operating range.

4. An electrical system, as claimed in claim 3, in which a commonpotentiometer is provided for securing and operating biasing potentialfor said tube, and a ballast resistance in the connection from saidpotentiometer to said auxiliary electrode.

5. The method of operating gas filled photoelectric tubes having a lightsensitive cathode, a

main anode, and an auxiliary positive electrode comprising applying apositive potential to the tube insufiicient to produce an independentglow discharge biasing the auxiliary electrode to a critical positivepotential close to the anode potential to secure a drooping shape of thesensitivity characteristic of the tube over a critical operating range.

6. In a gas filled photoelectric tube having a light sensitive cathode,an anode and a meshlike auxiliary electrode intermediate said cathodeand anode, the method of operation comprising applying a positivepotential to said anode insuflicient to produce an independent glowdischarge through the tube and biasing said auxiliary electrode to acritical potential close to the potential of said anode to secure evensensitivity of said tube over a critical operating range.

'7. In combination with a gas filled photoelectric tube having a lightsensitive cathode, an anode and an auxiliary electrode; means forapplying operating potential to said cathode and anode insufiicient toproduce an independent electric glow discharge; and further means forbiasing said auxiliary electrode to a predetermined potential forsecuring a critical operating range of substantially constantsensitivity of said tube to light variations.

HEINRICH H. GEFFCKEN.

